1. Field of the Invention
The present invention relates to a readout circuit for an electrically rewritable non-volatile memory device.
2. Description of the Related Art
FIG. 5 illustrates a conventional floating gate tunnel oxide (FLOTOX) non-volatile memory device. The conventional FLOTOX non-volatile memory device includes a single memory element 51 connected to a current load circuit formed of NMOS transistors 54 and 55 and a single dummy memory element 61 connected to a current load circuit formed of NMOS transistors 64 and 65, and converts a current generated by the memory element 51 and a current generated by the dummy memory element 61 into voltages. The converted voltages are compared with each other in a voltage comparator circuit formed of NMOS transistors 56, 66, and 70 and PMOS transistors 57 and 67, to thereby distinguish between 0 and 1 of data.
The memory element 51 has a binary value in the depletion state and the enhancement state. The voltage at an input node of the voltage comparator circuit on the memory element side changes depending on the difference in the state. Therefore, this voltage is compared with a reference voltage at an input node of the voltage comparator circuit on the dummy memory element 61 side, to thereby distinguish between 0 and 1 of data.
In general, in the FLOTOX non-volatile memory device, repeated rewriting of data leads to deterioration in film quality of a tunnel oxide film, resulting in a small threshold difference between the depletion state and the enhancement state. In a readout circuit required to read out data over the entire operating temperature range and the entire operating power supply voltage range, if the threshold difference becomes smaller as described above, it becomes difficult to distinguish between 0 and 1 of data, and it becomes impossible to distinguish between 0 and 1 of data in the operating temperature range and the operating power supply voltage range. In other words, a readout margin for distinguishing between 0 and 1 of data is reduced.